This invention relates generally to electronic measurement instruments having multiple measurement channels and in particular to a method of obtaining voltage isolation between each measurement channel of an electronic measuring instrument.
Test and measurement equipment such as voltmeters and oscilloscopes measure signals in a wide variety of applications. Many of these applications require measuring multiple signals at the same time using multiple measurement channels. Often, these multiple signals do not share a common ground and may be at substantially different reference levels, thus requiring voltage-isolation between measurement channels coupled to the multiple signals. For example, a measurement application that requires measuring an output voltage of a power supply while simultaneously measuring the power line input must be done with two measurement channels that are voltage-isolated from each other.
However, many measuring instruments have multiple measurement channels that share a common ground at the input terminals, making such commonly desired measurements as the previously power supply example very difficult The primary problem with measurement channels that are not voltage-isolated is in getting valid measurements. Interference between the multiple signals may result from the differential currents that flow through the common ground because of the different reference levels of the signals. It is also possible that the device being tested can be damaged through differential currents that can flow between the circuit nodes being tested. At high voltages, the common ground may "float" up to hazardous voltage levels to become a safety hazard to the operator who comes in contact with the common ground connection now unexpectedly at an unsafe voltage potential.
A solution to the common Found problem when no voltage-isolated measurement channels are available was to provide two measurement channels that can measure a signal differentially such that the voltage difference is not measured with respect to ground. Such differential mode measurements were difficult both to set up and to obtain signal measurements of acceptable accuracy. Furthermore, two measurement channels must be used to get one voltage measurement. Thus, a total of four input channels are required to obtain two simultaneous measurements of signals with different reference levels, making this solution relatively expensive and with only partially successful measurement results.
Voltage-isolated measurement channels that do not share a common ground connection have a number of advantages. First, ensuring an accurate measurement requires that signals between measurement channels not be contaminated by differential currents flowing through the common ground connection between the measurement channels. Furthermore, operator safety is a concern where a common ground connection between input channels that are not isolated may expose the instrument operator to a hazardous voltage source. Voltage-isolation requires coupling each of the input signals through a voltage isolation barrier which serves to pass the input signal while providing isolation between each of the measurement channels of the reference voltage level of each of the input signals. Achieving voltage isolation between each of the measurement channels within the measurement instrument thereby prevents unexpected connections to hazardous voltage levels via the common ground of the measurement instrument. Voltage-isolated measurement channels simplify the task of setting up a measurement because the user can simply couple each measurement channel to each signal, without regard for the reference voltage levels of each of the signals.
Modem designs of measurement instruments have incorporated digital circuitry to process the input measurements, typically convening the analog input signals into digital measurement values using analog to digital converters (ADC's) and saving the digital measurement values in digital memory for further processing and display. Such instruments include, among others, digital multimeters (DMM's) and digital storage oscilloscopes (DSO's). Converting the input signals to digital measurement values, a process called digitizing, has provided an opportunity to obtain voltage isolation between measurement channels by using ADC's in the voltage-isolated portion of the measurement channel and sending the digital measurement values across a voltage isolation barrier. Voltage isolation barriers for digital signals are commercially-available electronic components commonly referred to as opto-isolators or opto-couplers. A digital signal is coupled across the voltage isolation barrier within the opto-isolator, typically an air gap, using a light emitting diode (LED) which controls a bipolar transistor on the other side of the barrier. However, opto-isolators that provide sufficient data transmission speed tend to be costly and an opto-isolator would be needed for each digital data line. Therefore, it would be desirable to provide a method of providing voltage isolation between measurement channels using a method for coupling an analog input signal across a voltage isolation barrier.